Asphalt is manufactured by a variety of methods producing types and grades of different specifications. The properties range from soft and flowable to hard and brittle. They have found uses in coating, sealing, and adhesive applications in construction and industrial applications. These materials are thermoplastic in nature, that is, they need to be heated above their melting point in order to be applied. Various technologies have been developed to circumvent the practice of heating for application. They involve placing the asphalt in a fluid media, or vehicle, allowing the their application without the necessity of heating. They solidify or cure through the elimination, in most cases by evaporation, of the vehicle. The two most common vehicles are solvents and water.
The use of solvents requires solubility of the asphalt in the solvent, most often petroleum distillates are used as solvents. They produce blends that are commonly referred to as cut backs or road oils. They vary in viscosity and cure rate depending on the type of base asphalt, type of solvent, and composition.
The use of water as the fluid media requires the use of surface active agents (emulsifiers). This mixture produces a dispersion commonly called an emulsion. Asphalt emulsions consists of two types, those in which the asphalt is dispersed in an external phase which is water, and those in which the water is dispersed in the external phase which is asphalt.
Asphalt in water emulsions use surface agents that can be classified as anionic, cationic, and nonionic types. The dispersed phase of anionic and cationic types are charged while nonionics neither carry nor impart any charge. Anionic emulsions are produced from a variety of materials such as soaps of fatty acids and oils (oleic acid, tall oil), alkali soaps, metallic soaps, rosin soaps, and caseins. Cationic emulsifiers are produced from quaternary nitrogen compounds while non-ionic emulsifiers are produced from esters of alcohols such as glycol mono, di, and tri-laurates.
Water in asphalt emulsions use surface active agents that can be classified as oil soluble, inorganic powder, and non-soap types. Emulsions produced using oil soluble compounds utilize the solubility of the compounds in the external phase, which is asphalt in this case. Examples of these types of materials are lead tallate, calcium naphthenate, and other similar heavy metal salts of fatty acids such as fish oil acids, crude soya acids, petroleum sulfonic acids, etc. Emulsions produced using inorganic materials are based on the fact that they act as dispersants for water. Examples of the inorganic type are mainly fined powdered clays, preferably chemically modified clays, the most important being bentonite. Bentonite clay forms extremely colloidal gelatinous mixtures yielding asphalt dispersions of very small particle size. The clay is very hygroscopic in nature, holding water tenaciously bringing it into dispersion within the asphalt. Emulsions of this type are commonly referred to as clay emulsions. Examples of nonsoap types of materials are tannins, hydrous oxides, metallic hydroxides, lignins and alginates. Asphalt emulsions are discussed in Barth, Asphalt, Science and Technology, Gordon and Breech: New York, 1968, Chapter 7.
The choice of the above-described asphalt emulsions is dependant upon its particular application. The asphalt in water type are used in industrial applications and extensively in construction applications, predominately pavement construction and sealing. Cationic emulsions typically cure rapidly and often perform well with mixtures of aggregate, especially those with electronegatively charged surfaces. It is because of these reasons these cationic types are used almost exclusively in paving applications. Their drawback is that they have poorest storage stability in terms of settling and particle coalescence and that aggregation often cannot be reversed with agitation. Because of their charge, they are generally not compatible with fibers, fillers, or other particle type additives.
Anionic emulsions, on the other hand, are generally slower curing, have better storage stability (three to six months), and are compatible with fibers, fillers and other additives. Anionic emulsions having this type of versatility are often compounded with additives and used as coatings, sealants, crack fillers, adhesives in construction and industrial applications. Non-ionic asphalt emulsions are not common. These asphalt in water emulsions typically have asphalt solids of thirty to sixty five percent, the anionics generally being higher. These emulsions "break" or undergo the phenomena of irreversible particle coalescence, when subjected to a single freeze thaw cycle.
The water in asphalt type emulsions are used extensively in construction as paving sealers and water/damp-proofing materials, and, in industrial applications, such as roofing, as coatings and mastics. The most common of these types of asphalt emulsions are clay stabilized emulsions. These emulsions can vary in asphalt solids concentration depending on the application, but are generally in the range of forty five to fifty five wt. percent. One of the most unique characteristics of the clay emulsion is its theology, it can be described as plastic fluid displaying a distinct yield value. This property imparts non-sag characteristics which are unique. It ideal for the sloped roofing application or the vertical damp-proofing foundation application. The emulsion is extremely stable having shelf lives of two years or more and is compatible with a wide range of polymer latices, fibers, fillers, and other particulate additives. The emulsion will generally break when subjected to a freeze thaw cycle; however, it is a common practice for the addition of a glycol, to act as an antifreeze, imparting the tolerance for a few freeze thaw cycles.
The use of emulsions as compared to asphalt cutbacks is becoming increasingly popular as the product of choice for the non-heated applications. This is because of the dangers and hazards of solvent containing compositions to the environment and hazards with employee exposure to solvent in manufacturing and application. Storage stability, freeze thaw stability, limited compatibility with auxiliary additives, and low binder (asphalt solids) contents are deficiencies in current asphalt emulsions which hinder and often restrict their use.